Surge conditions occur in a centrifugal compressor when the inlet flow is reduced to the extent that the compressor, at a given speed, can no longer pump against the existing pressure head. At this point, a momentary reversal of flow occurs along with a drop in pressure head. Normal compression resumes and the cycle repeats. This causes a pulsation and shock to the entire compressor and piping arrangement. If left uncontrolled, damage and danger to the compressor could result.
All centrifugal compressors are supplied with characteristic and setpoint curves defining the zones of operation for the compressor. These compressor "maps" illustrate the surge area and the "stonewall" area of pumping limit of the turbomachinery. As shown in FIG. 1a, the surge limit line is plotted against a discharge pressure versus flow rate relationship. Taking into account no changes in speed, or inlet gas temperature the surge control line can be plotted with this equation. ##EQU1##
Three common forms of presently used surge control lines are shown in FIG. 1. The one position of this line is parallel to the surge limit line (FIG. 1a). To minimize recirculation, the surge control line should be set as close to the surge limit line as possible. Setting the control line with a slope less than that of the limit line (FIG. 1b) can lead to excess recirculation at high pressures, and surge at low pressures during stopping and startup. The third method is to select a minimum safe volumetric flow, and set a vertical control line (FIG. 1c). This can lead to excess recirculation at low pressures, and surge at high pressures. Many systems measure flow in the discharge without correcting for suction condtions. This gives maximum recirculation with minimum surge protection.
In the various surge controls, control is accomplished by opening a bypass valve around the compressor or blowing off gas to atmosphere to maintain minimum flow through the compressor. Since bypassing or blowing off gas wastes power, it is desirable to determine surge flow as accurately as possible to avoid bypassing fluid unnecessarily while maintaining safe operation. However, determining surge flow is often not a simple matter, but a complex one. Surge conditions can be approached slowly or quickly and thus situations may occur when the normal surge control loop opening the bypass valve opens the bypass valve too slowly to prevent a surge condition. Prior art systems used a second control loop for such emergency surge conditions to provide speedy and complete opening of the bypass valve. An example of such a control system having two separate control loops may be found in U.S. Pat. No. 4,142,838.
Clearly such prior art two mode control systems having two separate control loops were complicated, unstable, expensive, and required extensive coordination to properly switch between these two control loops. What was needed was a simple, single control loop which would provide control for both normal surge and emergency fast surge conditions.